The present invention generally relates to semiconductor laser driving circuits, and more particularly to a semiconductor laser driving circuit which drives a plurality of semiconductor lasers.
A semiconductor laser is used as a pump light source of an optical amplifier, a light source of a laser printer, a light source of a processing equipment and the like. When using the semiconductor laser as the light source, it is desirable to maintain the light emission quantity of the semiconductor laser constant. Normally, the light emission power of the semiconductor laser is controlled constant by use of an automatic power control (APC) circuit.
FIG. 1 shows an example of a conventional APC circuit. In FIG. 1, an APC circuit 101 includes a comparator 101, a resistor 102, and a current adjusting circuit 103. Vcc denotes a power supply voltage.
A light which is emitted to the rear direction of a semiconductor laser (laser diode) 110 is detected by a photodiode 111, and the photodiode 111 outputs a current which is dependent on the quantity of the detected light. This output current of the photodiode 111 flows through the resistor 102. Hence, a voltage V.sub.1 which is dependent on the power of the detected light is applied to one input terminal of the comparator 101. The comparator 101 compares this voltage V.sub.1 with a reference voltage V.sub.ref which is applied to the other input terminal of the comparator 101, and controls the current adjusting circuit 103 depending on a result of the comparison. Hence, a current which flows through the semiconductor laser 110 is adjusted by the current adjusting circuit 103, so that the light emission power of the semiconductor laser 110 becomes constant.
In the optical amplifier or the like, the required light emission power of the pump light source cannot be obtained by use of a single semiconductor laser. Hence, a predetermined light emission power is obtained by adding outputs of two or more semiconductor lasers. Even in such a case, it is necessary to control a sum of the outputs of the two or more semiconductor lasers constant.
FIG. 2 shows a conceivable semiconductor laser driving circuit for controlling an output sum of n semiconductor lasers constant. In FIG. 2, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. In FIG. 2, a coupler 150 obtains and outputs an summed output of n semiconductor lasers 110.sub.1 through 110.sub.n. In addition, an APC circuit 100.sub.i is provided with respect to each semiconductor laser 110.sub.i, where i=1, . . . , n.
In the conceivable semiconductor laser driving circuit shown in FIG. 2, one APC circuit 100.sub.i is provided with respect to each semiconductor laser 110.sub.i the light emission power of each semiconductor laser 110.sub.i is controlled to be constant, so as to control the output sum constant. For this reason, there is a problem in that an independent reference voltage V.sub.refi must be set with respect to each APC circuit 100.sub.i by taking into consideration the individual characteristic of each semiconductor laser 100.sub.i which deviates among the semiconductor lasers. However, it requires extremely troublesome adjustments in order to set the reference voltage V.sub.refi with respect to each APC circuit 100.sub.i.
On the other hand, even if one semiconductor laser 110.sub.j out of the n semiconductor lasers 110.sub.1 through 110.sub.n fails or its characteristic deteriorates for some reason, for example, information related to the failure or change occurring in the semiconductor laser 110.sub.j is not notified to the APC circuits which are provided with respect to the remaining semiconductor lasers. For this reason, even if only one semiconductor laser 110.sub.j fails or its characteristic deteriorates, for example, there is a problem in that the output sum output from the coupler 150 changes. For example, if one semiconductor laser 110.sub.j fails, the output sum output from the coupler 150 decreases by an amount corresponding to the light emission power of one semiconductor laser, and the reliability of the light source greatly deteriorates if this output sum is used as the output of the light source.